The First Hubble Detection of a Secondary Eclipse from a Rocky Exoplanet: the 5.4-hour planet TOI-2431 b

The work reports an HST/WFC3 G141 phase-curve observation of the ultra-short-period rocky exoplanet TOI-2431 b—a 5.4-hour planet—and the detection of its secondary eclipse in the near infrared. Combining this with the TESS optical eclipse is intended to constrain the planet's dayside brightness temperature, reflected-light contribution, albedo, and heat redistribution.

The secondary eclipse is detected in both the TESS optical band and HST/WFC3 G141, with planet-to-star flux ratios of 37 ± 10 ppm and 69 (+14/−13) ppm, respectively. Assuming negligible reflected light, a joint TESS+HST fit yields a dayside brightness temperature of T_b,joint = 2524 (+77/−84) K and a ratio R_joint = T_b,joint/T_p,max = 0.980 (+0.032/−0.034), consistent with the zero-albedo, no-redistribution limit T_p,max = 2574 (+36/−35) K.

The HST phase curve is consistent with zero nightside emission and no measurable hotspot offset, which points to inefficient day-night heat distribution. Under this thermal-dominated interpretation, TOI-2431 b lies at the zero-albedo, no-redistribution limit and contrasts with previously observed lava worlds. However, at optical and near-infrared wavelengths, thermal emission is degenerate with reflected light. For a geometric albedo of A_g = 0.1, reflected light would contribute about 11 ppm in each bandpass, lowering the joint brightness temperature to T_b,joint(A_g = 0.1) = 2407 (+91/−102) K.

Longer-wavelength observations are needed to isolate the thermal component. Planned JWST/MIRI LRS observations are expected to substantially reduce this degeneracy and provide more direct constraints on heat redistribution and possible silicate-atmosphere spectral features. The result marks the first secondary-eclipse detection of a rocky exoplanet using HST and establishes TOI-2431 b as a benchmark target for studying lava-world properties.